In a battery, a separator is used to separate a positive electrode and a negative electrode, to prevent a short circuit therebetween, and further, to hold an electrolyte thereon and enable a smooth electromotive reaction.
Recently, a space allotted in electronic equipment for the battery has become smaller, due to the need for miniaturization and weight-saving. Nevertheless, the performance requirement for such a smaller battery is the same as or higher than that for a conventional battery, and therefore, it is necessary to enhance the capacity of the battery, and to increase the amounts of active materials in the electrodes. Accordingly, a volume allotted in the battery for the separator must be reduced. Although a thin separator having a thickness of 0.15 mm has been proposed, a thinner separator having a thickness of 0.1 mm or less is desired.
As such a separator, Japanese Unexamined Patent Publication (Kokai) No. 2002-124239 discloses “a battery separator consisting essentially of a nonwoven fabric having a substantially unilayered structure, wherein an apparent total surface area of fibers per a surface density of the nonwoven fabric is 20 m2 or more, a thickness of the nonwoven fabric is 0.1 mm or less, a uniformity index of the nonwoven fabric is 0.15 or less, and the nonwoven fabric contains superfine fibers having a fiber diameter of 4 μm or less” (for example, claims 1, 3, and 4).
The battery separator disclosed in JP 2002-124239 contains superfine fibers having a fiber diameter of 4 μm or less. Therefore, the separator has a wide apparent total surface area of fibers per a surface density, and can provide a separator having an excellent electrolyte-holding capacity, even though the separator is thin. In this connection, JP 2002-124239 discloses that the superfine fiber may be formed from an island component remaining after removing a sea component from an islands-in-sea type composite fiber. It is disclose that the separator disclosed in JP 2002-124239 preferably contains high-modulus fibers having a Young's modulus of 50 cN/dtex or more, because a short circuit caused by a tear in the separator by an edge of an electrode or a penetration into the separator by a flash at the electrode, when the separator is wrapped around the electrodes, will be avoided.
Japanese Unexamined Patent Publication (Kokai) No. 10-251953 discloses “a nonwoven fabric characterized in that, at least, superfine fibers A generated from removal-type dividable fibers capable of generating superfine fibers A having an average fiber diameter of 3 μm or less by removing one or more resin components; physically dividable fibers capable of generating superfine fibers B by dividing the fibers by a physical action; and the superfine fibers B generated from the physically dividable fibers are entangled and unified” (claim 1). JP 10-251953 further discloses that the nonwoven fabric may contain adhesive composite fibers (paragraph [0036]), and that the nonwoven fabric may be used as a battery separator (paragraph [0051]).
However, when the nonwoven fabric is used as a battery separator having a thickness of 0.1 mm or less, a short circuit caused by a tear in the separator by an edge of an electrode or a penetration into the separator by a flash at the electrode, when the separator is wrapped around the electrodes, easily occurs. Further, the nonwoven fabric prepared via the entangling treatment has a large maximum pore size, and thus a short circuit caused by a movement of electrode active materials in a battery is liable to occur when the battery is used.
Japanese Unexamined Patent Publication (Kokai) No. 2002-180330 discloses “a drawn composite fiber generated by drawing undrawn composite fiber spun by a melt spinning method and having a core component of a crystalline propylene based polymer and a sheath component of an olefin polymer other than the crystalline propylene based polymer, wherein a breaking strength of the drawn composite fiber is more than 5.74 cN/dTex, an elongation percentage thereof is 30% or less, and a Young's modulus thereof is 43.1 cN/dTex or more” (claim 1). JP 2002-180330 further discloses that the drawn composite fiber may be used as a battery separator (paragraph [0001]).
However, JP 2002-180330 does not disclose fibers other than the drawn composite fiber in use as the battery separator. The battery separator could not be produced with the drawn composite fiber alone.